Personality theories have long tried to use self-report scales, designed to index hypothesized individual differences in neural reactivity, in order to predict cognition, behavior, and affect. With the advent of functional magnetic resonance imaging (fMRI) it is now possible to quantify these differences much more directly than we have been previously able with personality scales. We provide evidence from psychiatric disorders, neurophysiology, and computational neuroscience that together suggest the anterior cingulate cortex (ACC) may play an important role in neuroticism and the social cognitive outcomes conceptually associated with neuroticism. Using a social cognitive neuroscience approach (Lieberman, 2000; Ochsner & Lieberman, 2001), we propose to look at the ways in which individual differences in ACC reactivity shape social cognition and how social factors, in turn, can shape ACC reactivity. We hypothesize that individuals with highly reactive ACC's will engage in more self-focused processing, social comparison, and self-doubt which together should lead to lower self-perceived status. Additionally, we hypothesize that social threat increases ACC reactivity while positive social contact decreases ACC reactivity. Finally, we predict that the effect of these social factors on ACC reactivity will be greater for individuals with highly reactive ACC's, compared to those with less reactive ACC's. We propose to test these hypotheses across four experiments combining fMRI and social psychological methodologies to assess: (1) the relation of dispositional ACC reactivity to self-awareness of arousal, (2) the relation of dispositional ACC reactivity to frequency of self-awareness, social comparison, and self-doubt in a daily diary study, (3) the effect of social threat on ACC reactivity, and (4) the effect of positive social contact on ACC reactivity to painful stimulation.